Degradation of isolated grass mesophyll, epidermis and fibre cell walls in the rumen and by cellulolytic rumen bacteria in axenic culture

Abstract

The degradation of cell walls of mesophyll, epidermis and fibre cells isolated from leaves of perennial and Italian ryegrass within the sheep rumen or by selected strains of rumen bacteria in vitro, was followed by estimation of dry matter loss, or loss of neutral sugar residues. Primary cell walls (mesophyll and epidermis) were fully degraded within 12 h in the rumen, while the more heavily lignified fibre cell walls showed only a 40% loss of dry matter over the same period. Neutral sugar residues were lost at a common rate from walls of all three cell types. Incubation of cell walls with cellulolytic bacteria showed that the extent to which cell walls were attacked was constantly ordered (epidermis > mesophyll > fibre). The rate of degradation of cell walls was less in axenic culture than within the rumen. Greatest weight losses were produced by Ruminococcus albus, followed by Bacteroides succinogenes, with Ruminococcus flavefaciens effecting the least change, regardless of the nature of the cell wall provided as a substrate. Xylose was more readily lost from primary cell walls than glucose during the early stages of attack, but both were lost at a common rate from fibre cell walls. Dry matter losses produced by the hemicellulolytic strain, Bacteroides ruminocola, were limited even after extended incubation. Electron microscopy indicated that R. albus was less commonly attached to cell walls than were the other cellulolytic strains, although evidence of capsular material was present. Bacteroides succinogenes was seen with an extensive capsule which enveloped clusters of cells, forming micro‐colonies in association with the plant cell wall. Vesicle‐like structures, commonly associated with the cellulolytic bacteria R. albus and B. succinogenes, were found on comparatively few occasions in this study.